The present invention relates to an improved method and apparatus for use in the photometric testing of high intensity discharge lamps and luminaires. Photometry, the measurement of light intensity, light energy and brightness relies upon a comparative analysis between the subject lamp or luminaire, and a "working standard" or test lamp having a published rated level of lumen output. Photometric measurements taken from various angular zones surrounding the subject lamp or luminaire are compared with the known lumen rating of the test lamp, thereby providing a quantitative analysis of the photometric qualities of the subject lamp or luminaire.
With the advent and widespread use of "relative photometry" it is no longer important that the lumen output of the test lamp actually duplicate its published lumen rating during testing. Since the test lamp is used as a radiant intensity emitter, it is known that the lamp will radiate in all directions in true proportion to its "normal" emission regardless of the level of its lumen output of wattage output. Therefore, as long as the test lamp in use has also been used to calibrate the sensor of the photometer, the actual candela values of luminous intensity can be pro-rated from the raw intensity readings taken during testing, in relative proportion to its published values. It is of critical importance, however, that the test lamp duplicate the structural and electrical details of the subject lamp or lamps intended for use with the subject luminaire within acceptable tolerances. Accurate duplication of all electrical and structural details is necessary to ensure that the radiant emission pattern of the test lamp remains in true proportion to the radiant emission pattern of the subject lamp.
The increased use and acceptance of high intensity discharge (HID) lamps, in varied commercial and industrial applications, has created numerous problems for photometric testing laboratories. Many of the problems being encountered by the laboratories are created by the very nature of the HID lamp. For instance, it is difficult to maintain a stable lumen output from an HID lamp during the test-taking period because a variety of factors, such as lamp orientation and changing environmental conditions can critically affect the stability of the arc stream. It is important to maintain the desired spacial orientation of an HID lamp during testing since the arc stream of the HID lamp tends to rise against gravity and can actually engage the walls of the arc tube if the lamp is oriented in an undesirable position. Further, it is desirable to maintain thermal equilibrium of the walls of the arc tube as variations in temperature along the surface of the arc tube will result in lumen instability. Such variations in thermal equilibrium are commonly caused by poor lamp orientation and instable environmental conditions. For instance, simply positioning an HID lamp in a drafty environment can cause critical instability in the lamp's lumen output.
Problems are also created for photometric testing laboratories by the excessive amounts of warm-up time, commonly 1-3 hours, needed to stabilize the lumen output of an HID lamp. Further problems are encountered in testing the photometric qualities of HID lamps and luminaires because of the need for heavy, expensive ballasting and other electrical control systems which HID lamps uniquely require for start-up and stable maintenance of lumen output. Yet other problems have been encountered when testing low output HID lamps. Such low output lamps create low sensor signal strength and the inherent electrical "noise" of the measurement equipment interferes with the ability to accurately measure the lamp's photometric qualities. In all, while HID lamps have many unique qualities which make them durable and desirable in many commercial and industrial applications, the photometric testing of such lamps and luminaires can often be a time consuming and frustrating chore in which many variables and inconsistencies affect the accuracy of the test results.